Controls on species distribution and biogeochemical cycling in nitrate-contaminated groundwater and surface water, southeastern Australia

A detailed study of groundwater and surface water nitrate over four seasons across an area of varied landuse provided insights into the mechanisms that underlie accumulation and transport of nitrate. High nitrate concentrations found in a significant percentage of surface water and shallow groundwater samples are due to anthropogenic contamination. Statistics (PCA, ANOVA, parsimonious model and general linear regression) were used to explore the relationship between NO3- and land use, and confirmed that areas of high NO3- concentration are associated with dairy pasture and horticulture. Seasonally, NO3- levels are greater during winter, the wettest part of the year. Values of delta N-15 showed that most nitrate is sourced from livestock waste, with a smaller contribution from synthetic fertilizer. Direct wash-off of animal waste from dairy farms results in higher NO3- concentrations in surface water than in groundwater. Denitrification is an important NO3- attenuation mechanism which reduces NO3- to NH4, as demonstrated by the PCA analysis, which showed positive correlation of NO3- concentrations with dissolved oxygen and negative correlations with NH4+, Fe-2(+) and Mn2+; the latter two species may act as the electron donors necessary for reduction of NO3-. The often high NO3- concentrations in shallow groundwater are decreased by denitrification, which can occur at relatively shallow depths (<3 m). The relatively small NO3- concentrations in deeper groundwater are due partly to denitrification, but more to originally lower NO3- concentrations, as the age of deeper groundwater shows that it was recharged before agriculture was established in the study area. Overall, the study demonstrates the usefulness of hydrogeochemical characterisation and multivariate statistics in the evaluation of impacts of agricultural land-use on regional N cycling. In particular, the results show that efforts to mitigate NO3- pollution from farms should concentrate more on wash-off of animal waste than the contribution of nitrogenous synthetic fertilizer. Crown Copyright (C) 2020 Published by Elsevier B.V. All rights reserved.